This invention relates to the generation and amplification of radiation by stimulated emission. Devices for generating radiation by stimulated emission are commonly termed optical masers or lasers.
Lasers are commercially available which employ the principle of stimulated emission of radiation to generate monochromatic light radiation. The principles by which this type of device operates are disclosed in U.S. Pat. Nos. 2,929,922 to Schawlow et al, and 2,879,439 to Townes, and elsewhere in literature and are generally well understood. Briefly, a lasing medium, consisting of atoms or molecules, is pumped with radiation from an external source, such as a light source, in order to produce a population inversion in which the ratio of the total number of atoms or molecules in a higher energy level to the total number of atoms or molecules in a lower energy state is greater than the same ratio when the medium is in a state of equilibrium. The atoms or molecules in the higher energy level spontaneously decay to one or more lower levels, resulting in the emission of radiation of characteristic frequency. The spontaneous decay of atoms or molecules in turn stimulates the decay of other atoms or molecules in the higher level, and, so long as the population inversion is maintained, this stimulated emission of radiation continues to occur and results in the generation of coherent light having a wavelength determined by the characteristic frequency.
There are many potential applications for lasers which have yet to be exploited due to severe limitations inherent in known devices. For example, in the medical fields of microbiological analysis and surgery, lasers have many potential uses and applications. Ideally, when used as an analytical tool for microbiological analysis, the laser should ideally comprise a low power device having extremely high resolution and tunability over a relatively wide range of wavelengths. Currently available lasers used for this purposes, however, typically possess a lower limit on the emitted spectra of about 3,000 angstrom units. This limitation precludes the exploration of many organic compounds having spectra lying below 3,000 angstrom units and neural filaments whose size is in the 100-800 angstrom range. When used as a surgical tool, on the other hand, the laser should ideally comprise a device capable of generating radiation in the range from around 2500 angstroms to one micron, i.e., those wavelengths for which the human tissue has a special affinity, at relatively high power continuously variable from about 10 to about 100 watts continuous wave power with extremely high power stability. While carbon dioxide and argon ion lasers, for example, are known which are capable of generating radiation having up to 100 watts continuous wave power, such devices do not possess the precise power stability required and are further limited by the available wavelengths of the radiation generated.
In the communications field, lasers are theoretically well suited for both modulated carrier wave and pulse code modulated signalling applications that have not been extensively employed to date due to similar limitations: viz. power instability and the unavailability of radiation of specific wavelengths at which optimal transmission through the atmosphere, water or other media is attained. In addition, mobile signalling applications are presently impractical with known high power lasers, such as carbon dioxide gas lasers, due to their relatively large size and sensitivity to shock.
In addition to the above deficiencies, all known lasers are relatively inefficient in operation, with a maximum efficiency of 15-20 percent being typical.
Efforts to provide lasers which do not suffer from the above known disadvantages have included the suggestion of designing electron impact excitation lasers in articles by W. R. Bennett and G. Gould in Applied Optics Supplement, Volume 2, pages 3 and 59, respectively; direct electronic ion excitation lasers in articles by P. K. Tien et al, in Physical Review Letters, Volume 7, page 159; and direct electron excitation of molecular states in articles by R. W. Waynant Physical Review Letters, Volume 28, page 533 and R. T. Hodgson et al in Physical Review Letters, Volume 28, page 536 and Physics Letters, Volume 38A, page 213. To date, however, such efforts have not led to the successful development of a monoenergetic electron impact laser capable of utility in wide variety of application, such as those noted supra.